In a cooking such as an electrical oven, small kitchen range and the like, an A.C. power source which is connected to house wiring has been employed. In many cases, a triac has been used to control an electrical heater mounted on the heater device since the triac causes no great noise, and at the same time, makes it possible to simplify the heater control circuit. In this instance, the triac is electrically connected in series with the electrical heater. The circuit in which the triac is in series with the electrical heater is further connected in series with an A.C. power source by way of a plug. In this situation, a surge current induced at the time of, for example, lightning, may flow through the house wiring and be drawn to the heater circuit of the device through the plug, thus causing damage to the triac due to the high voltage induced by the surge current.
In order to protect the device against surge voltage, a manual switch is connected to the device to make and break a conductive path toward the device, especially in the cases in which the device employs a house wiring rated at 200 volts. However, the cooking device is usually connected to the power source by way of the plug irrespective of whether the device is used or not. In this situation, it is desirable to cope with lightning without operating the manual switch each time the device is used.